International Journal of Mechanical Engineering and Applications 2014; 2(2): 25-30 Published online April 20, 2014 (http://www.sciencepublishinggroup.com/j/ijmea) doi: 10.11648/j.ijmea.20140202.11 Dimensionless criteria energy dissipation of dynamically heating surfaces Fikret Alic Department of Thermotehnics, Faculty of Mechanical Engineering Tuzla, Tuzla, Bosnia and Herzegovina Email address: fikret.alic@untz.ba To cite this article: Fikret Alic. Dimensionless Criteria Energy Dissipation of Dynamically Heating Surfaces. International Journal of Mechanical Engineering and Applications. Vol. 2, No. 2, 2014, pp. 25-30. doi: 10.11648/j.ijmea.20140202.11 Abstract: In this paper, a discussion of results is presented for the dimensionless analysis of generating irreversibility of vessels in which mixing and heating of fluid are done simultaneously. In the first case, the impeller inside the mixing vessel is the heating body, and in the second case heating body is a fixed ring and the impeller inside the vessel provides only mixing of the fluid. The paper presents a comparative analysis of typical irreversible dimensionless parameters in both cases. A mathematical model is established to describe the thermal-hydraulic irreversibility of heating-mixing vessel which indirectly gives them the ability to minimize and maximize the efficiency of such a system. Also, the paper established typical relations between the dimensionless entropic values and power number, as for the water heater impeller and also for impeller, combined with a heating ring, which will enable comparison of the required power numbers Keywords: Entropy Number, Heated Impeller, Mixing of Fluid, Thermal and Hydraulic Irreversibility, Ring, Vessel 1. Introduction Heat transfer rates in agitated vessels are very important for many applications, and there are many papers and studies on heat transfer in mixing vessels [1]. Many studies and analyzes of agitated vessels are based on finding a correlation between the geometry of vessel and impeller, the physical properties of the agitated fluid, and correlations between the power and Reynolds number, etc. [2-3]. Many types, designs and sizes of agitators are used to mix fluids in vessels. Many studies and analyses discuss theoretical aspects and effects of fluid mixing in different vessels. For example, technical applications of various impeller types are discussed [3-8]. Highly viscous fluids are normally mixed by screw or helical ribbon impellers, which have relatively large convective heat transfer coefficients [6-10]. Installation of an electrical heating element inside an impeller blade [11] extends the overall role of the blade from only mechanical to included thermal effects due to heat transfer with the fluid. For a given blade geometry, the heating rate depends on the temperature gradient between the blade and the fluid and the convective heat transfer coefficient. Increasing the speed of rotation increases the heat transfer coefficient, decreases the blade surface temperature and affects the fluid temperature as it increases to reach thermal equilibrium with the temperature of the impeller blades. Increasing the electrical current intensity through the heater increases the blade temperature and establishes a steeper temperature gradient between the heated blades and the fluid, thus increasing the fluid heating rate. When heated blades rotate, irreversibility develops due to hydraulic friction, blade geometry and thermal irreversibility, which is caused by the existence of a temperature gradient between the heated blades and the fluid [12]. Therefore, considering the impeller and fluid as a closed system, the total entropy of this system is the sum of the thermally and hydraulically generated fluid entropy and the thermal entropy generated in the heat source, i.e., the heated blades. The rates of hydraulic and thermal irreversibility for constant convective impeller surfaces depend on the rotation speed of the impeller, the temperature differences between impeller and the fluid, the impeller geometry and the mass and physical properties of the fluid. If the fluid temperature in a closed vessel increases continuously and the impeller surface temperatures remain constant, then the temperature difference between the impeller and the fluid decreases, and, in general, transient convection occurs. Transient convection induced in this way generates transient fluid entropy before fluid entropy is thermally generated. An analysis of the causes for increases in the fluid entropy and the effects of heating impeller indicate possible methods for minimizing the losses caused